Hollow articles and method of manufacture

ABSTRACT

A hollow jewelry pendant or bead formed by investment casting is disclosed. The method of manufacture consists of producing a mold usually consisting of metal, silicone or rubber containing a cavity for injecting an inner soluble core. A geometric lock is also formed as part of the injection core which will orientate the core both laterally and radially. This core will define the inner surface or hollow portion of the pendant as well as form the bail or hole through which the chain will slide. A second mold is produced usually consisting of metal, silicone or rubber containing a cavity into which the soluble core will be placed with the lock positioning the core so as to insure proper orientation. This is critical for consistent wall thickness and matching of design features. The cavity also allows for a wax or plastic material suitable for investment casting to be injected around the soluble core. The part consisting of the pendent or bead and soluble core is then removed from the mold and the core is dissolved using the appropriate water or chemical solution.

CROSS REFERENCE TO RELATED APPLICATION

This application is a non-provisional of U.S. Application Ser. No. 61/061,223 filed on Jun. 13, 2008, titled Hollow Articles and Method of Manufacture which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to the production of a hollow jewelry article such as a bead or pendant by investment casting.

There are several methods of producing jewelry articles for mass production. Since precious metals are frequently used in the production of jewelry, material costs form a significant portion of the production costs. Since the reduction of the metal used will usually reduce the final selling price, for certain markets' making a lighter or hollow piece is often a desirable trait. Hollow pieces also allow for larger pieces that do not feel excessively heavy or cumbersome.

Several production methods are often employed for the manufacture of hollow pieces including; electroforming, casting and die striking or stamping. All of these methods have relative advantages and disadvantages. Electroforming allows for some of the lightest pieces but because of the extremely thin wall thickness pieces are prone to denting during normal wear. Structural integrity can be problematic or impossible for certain designs particularly those incorporating stone settings. Striking or stamping addresses some of these problems but high initial tooling costs and geometry limitations restrict design options and may require subsequent assembly, a costly procedure.

While not necessarily the best manufacturing method for every situation, casting gives the greatest freedom design and lowest initial start up, modeling or tooling costs.

BRIEF SUMMARY OF THE INVENTION

It is the object of the invention to provide for a method to produce a hollow pendant or bead in order to meet two objectives. First to allow for less material usage in the product and secondly to provide for adequate structural integrity so as to hold up to extended wear as well as allow for stone settings if so required by the design.

In accordance with this method two molds are made. The first providing for a cavity into which a soluble core or insert will be injected which will form the hollowed out portion of the pendant or bead.

The second mold will also provide a cavity similar in shape to the first only slightly larger. Part of this cavity will allow for the soluble core to be inserted into the mold and positioned accurately by a unique shaft and geometric lock system. This allows design features in the pendant or bead to be accurately matched up with the respective features in the soluble core. Into this second mold and around the soluble core is injected a wax, plastic or other material suitable to investment casting. The combined pieces are removed from the mold and the soluble core is dissolved by water or other chemical means. This leaves a hollow pattern ready for investment casting.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIGS. 1 a, 1 b are cross section views showing relative properties of an example bead or pendant in both solid and hollow forms.

FIG. 2 perspective view of both halves of soluble core mold.

FIG. 3 perspective view of both halves of soluble core mold with soluble core injected.

FIG. 4 a soluble core after removal from mold.

FIG. 4 b soluble core after clipping sprue.

FIG. 5 perspective view of both halves of pendant/bead mold.

FIG. 6 perspective view of both halves of pendant/bead mold with soluble core placed.

FIG. 7 perspective view of both halves of pendant/bead mold with soluble core place and wax pendant/bead injected around core.

FIG. 8 perspective view of wax pendant/bead pattern containing soluble core after removal from mold.

FIG. 9 perspective view of final wax pendant/bead pattern after removal of soluble core.

DETAILED DESCRIPTION OF THE INVENTION

Additionally for casting the purpose of making the piece hollow is not strictly for the purpose of material savings, although important, it is also for making a piece that can be more successfully cast. As is well known in by casting practitioners, casting pieces with large solid cross sections pose particular problems. A spherical or oval bead is probably the most perfect example to illustrate that problem. In the casting process molten metal is injected into a cavity formed from investment material. The cavity is formed from a wax, plastic or similar material around which the investment material mixed into a slurry is poured and vacuumed to remove air from cavities and remove air bubbles mixed into the slurry.

Once the investment has solidified the “wax pattern” is “burnt out” to form the cavity into which the molten metal is injected. The metal is injected via a “sprue”, a connecting channel through which the molten metal moves from the outside into the finished piece. As the metal moves into the mold it begins to freeze and since most metals shrink in volume as they move from the liquid state to the solid state, metal must be able to continue to “feed” the shrinking metal in the piece in order to avoid what is commonly referred to as shrinkage porosity as the piece solidifies. As the metal freezes it does so from the outside in, if the sprue is too small it becomes choked off by solidifying metal and is unable to continue to feed the jewelry piece resulting in a porous casting. The way to compensate for this is to proportionately increase the spure size as the cross section of the jewelry piece increases. In particular with a spherical or oval piece in which the volume to surface ratio is high this poses difficult problems. To overcome this problem the sprue size would have to become ridiculously large. This tends to run counter to what is preferred as large sprues become more labor intensive to remove and finish as well as much more disruptive of surface designs.

Example FIG. 1 a is a solid bead with the following parameters:

bead diameter: 14 mm

drill hole: 2 mm

volume: 1392.88 mm³

surface area: 696.72 mm²

ratio v/a: 2.0 (1.999)

max. solid cross section: 6 mm

weight in 14 k: 18.65 grams

Example #1 b is a hollow bead with the following parameters:

bead diameter: 14 mm

drill hole: 2 mm

wall thickness: 0.8 mm

volume: 434.19 mm³

surface area: 1096.12 mm²

ratio v/a: 0.4 (0.396)

max. solid cross section: 0.8 mm

weight in 14 k: 5.81 grams

Bead 1 b has a mass of only 31% of 1 a, 5.81 grams compared to 18.65 grams, providing a significant materials savings. Maximum cross section is reduced from 6 mm to 0.8 mm (20 ga.). The piece also retains significant structural integrity adequate for stone setting as well as reducing the weight from uncomfortably heavy to a pleasing weight. This is a particularly important issue for pendants or beads with large cross-sectional areas much more so than in rings where the difference in volume to surface area is not nearly so great.

A unique feature of the piece is that the ports through which the soluble material is removed actually forms a functional part of the piece and are not just a useless artifact of the process. In all pieces the bail or areas the chain slide through is the primary means for removal of the soluble material. In some designs where stone settings are important, the pilot hole beneath the stone also becomes a means for soluble material removal. In all cases the resulting piece shows no left over unusable ports not necessary for other functional parts of the piece.

The pendant or bead has the following specifications. It is hollow having a volume to surface area ratio of less than 1 to 1. The wall thickness will allow sufficient integrity for normal wear and tear as well as the necessary structural integrity for stones to be set if required. All ports allowing for removal of soluble material form integral design features such as but not limited to a bail, chain slide or pilot holes for stone setting.

Referring to FIGS. 2 and 3, the method of manufacture requires two molds of metal, rubber, silicone or other material to be made. The first mold 1 made of two halves contains a suitable number of locks which may include a male portion 2 and a female portion 3 so that when the mold is closed it will accurately align the two halves. In this mold is a cavity representing the interior hollowed area of the pendant or bead 4. Usually this is an offset to the interior of the outside surface of the pendant or bead. Connecting this interior section is a cavity for the shaft or sprue 5. This shaft will continue through the bead and connects on the other side to a cavity that may form the geometric lock 6. The two halves of the mold are closed being properly aligned by the mold locks male portion 2 and female portion 3. Into this cavity is injected a soluble material such as a soluble wax 7. Once the injected soluble material has solidified it is removed.

Referring to FIGS. 4 a and 4 b, the molded wax has a surface defining the interior of the pendant or bead 10. The surface of the core accurately follows the design pattern of the outside surface of the finished piece 13. a shaft or sprue piercing the interior 12 and continuing to a geometric lock 11 which will accurately position the soluble core both laterally and radially in the second mold. The shaft or sprue of the soluble core is then clipped 14 to allow proper fitting into the second mold. Notable is the junction of the sprue with the core of the bead 15. This is the area that will ultimately form the bail or chain slide as well as the removal of the soluble core.

Referring to FIGS. 5 and 6, the second mold 16 comprises two halves contains a suitable number of locks comprising a male portion 17 and a female portion 18 so that when the mold is closed it will accurately align the two halves. In this mold is a cavity representing the exterior surface 19 of the finished pendant or bead.

There is also a channel joining the cavity to the exterior of the mold 20 this will form the sprue allowing the injection of the wax, plastic or other suitable material around the soluble core as well as later injection of molten metal during the investment casting process. Also in this mold is a cavity intersecting the previously discussed cavity 19 allowing the placement of the soluble core leaving space for the shaft or sprue 21 and the geometric lock 22 allowing for the accurate placement of the soluble core both laterally and radially. Also in this cavity are any design features or stone settings 23.

Into this mold as described is placed the soluble core locked into proper placement and orientation by the geometric lock 24.

Referring to FIGS. 7, 8, and 9, the two halves of the mold are closed being properly aligned by the mold locks male portion 17 and female portion 18. The core is now suspended by means of the soluble shaft or sprue and into the mold around the core 25 is injected a wax, plastic or other suitable material 26 forming the pattern of the pendant or bead needed for investment casting. The combination pattern 27 and soluble core 28 is removed from the mold. The soluble core is dissolved in water or suitable chemical to leave only the material needed for investment casting (FIG. 9) This comprises the hollow pattern of the pendant or bead 30 with the appropriate design which may include stone settings and pilot holes 31 as well as a bail or chain slide 32. Attached to the pattern is the sprue 33 allowing for proper investment casting.

While the method described pertains to the production of a hollow pendant or bead it may be utilized in a broad range of products and disciplines using a wide variety of materials. For descriptive purposes only, the method of manufacture will utilize a design of a hollow pendant or bead with decorative pattern which may or may not include setting for stones. 

1. A method of manufacturing a hollow article formed by investment casting comprising the steps of: providing a first mold comprising a first half and a second half; forming a soluble core that will define the interior of the article; the soluble core comprising a geometric lock to accurately position the soluble core both laterally and radially positioning the soluble core in a second mold defining the exterior of the article; and Injecting a molten wax, plastic or suitable material around soluble core.
 2. An article manufactured according to the method of claim
 1. 3. The article of claim 2, wherein the article comprises a pendant or bead.
 4. The article of claim 2, wherein ports used to inject and remove the soluble core also form functional features of the article.
 5. The article of claim 2, wherein the article has a material volume to surface ratio less than 1 to
 1. 